Google Says Most Invalid Clicks Come From Publishers

Google AdWords representative, Laura, said in a Google AdWords Help thread that the main reason there are "invalid clicks" on ads is due to publishers.

Yes - Google said that "usually" invalid clicks come from AdSense publishers. Either the publishers are:

(1) Clicking on their own ads or;

(2) Telling their friends and users to click on their ads.

Google's Laura said:

I don't think there's any connection between AdWords suspensions and AdSense ones. AdSense "invalid clicks" suspensions are usually just that: publishers clicking on their own ads or encouraging their visitors to do so. If you think you didn't violate that policy, contact AdWords support.

So there you have it - usually those invalid clicks are AdSense publishers fault.